The electron gun of FIG. 1 which was developed by Matsushita Electric Corporation of Japan and the electron gun of FIG. 2 which was developed by NEC company of Japan are typical dynamic focus electron guns. The common characteristics of these two electron guns lie in the fact that the electron beams are vertically elongated by means of a dynamic quadrupolar lens, thereby compensating for the degradation of the beam spot characteristics caused by the distortion of the deflecting magnetic field.
In the case of the electron gun of Matsushita, vertical and horizontal blades BV, BH for forming a quadrupolar lens are disposed at both the beam exiting plane 3P of an electrode G3 to which a static focus voltage is applied and at the beam entrance plane 4P of an electrode G4 to which a dynamic focus voltage is applied. The bladesd BV, BH are installed projecting from the respective planes toward the other electrode surrounding the respective red, green, and blue (R.G.B.) electron beam passages.
In the case of the electron gun of NEC company, vertically elongate beam passing holes G3H and horizontally elongate beam passing holes G4H are disposed at the beam exiting plane 3P of an electrode G3 to which a static focus voltage Vf is applied and at the beam entrance plane 4P of an electrode G4 to which a dynamic focus is applied voltage Vd.
In these two electron guns, a parabolic dynamic focus voltage Vd synchronized with the vertical and horizontal scanning signals is applied to the electrode G4. Therefore, the electron beams are vertically elongated when the electron beams are scanning the peripheral portions of the screen, that is, during the time when the electron beams are deflected at a large angle by the deflecting yoke, with a large astigmatism. Therefore, when the vertically elongated electron beams land on the screen after passing through the deflecting magnetic field, they form approximately circular spots. As a result, beam spots of a uniform shape are distributed over the whole surface of the screen, thereby greatly improving the quality of picture.
In such electron guns having the above described advantages, a dynamic electric field is formed between pairs of mutually opposingly facing electrodes, and therefore, the manufacturing process for the electron gun requires high precision. Dynamic electric fields are established between pairs of electrodes having certain potential differences, and therefore, the intensities of the electric fields are very sensitive to the dimension of the gaps between the pairs of the electrodes and are liable to be varied by variations in the dimensions of the gaps.
In the case of the electron gun of NEC company, there is a likelihood that the uniformity of the field intensity can be impaired by by non-planar deviations of the beam exiting plane 3P of the electrode G3 and of the beam entrance plane 4P of the electrode G4. In the case of the electron gun of Matsushita, the field intensity can vary depending on the assembly precision of the vertical blades BV and the horizontal blades BH. Further, in the case of the electron gun of Matsushita, the vertical and horizontal blades are closely spaced in a rectangular relationship surrounding the beam passages. Therefore, arcing might occur due to the potential differences.